The present invention relates to the measurement of levels, phase, interfaces, and density of substantially non-hydrogenous process materials using neutron backscattering for measurements.
The use of thermal neutron backscatter, thermal neutron transmission, fast neutron transmission, and various gamma radiation techniques for process measurements have long been known. For example, the assignee of the present application, Kay-Ray Sensall Inc., of Mount Prospect, Ill., has level and interface measurement systems using radiation sources in detection, such as their Model 4800X Level System, the Model 4760 Level System, the Model 4160 Neutron Level/Interface Measurement System, and the Model 3660 Density Measurement System. These systems use radiation principles, having sources of radiation and detectors, which provide an output indicating a level of process material. The detection circuitry senses the output and uses it for control of a process, for example, with microprocessor control and two or four wire transmitter systems.
Fast or high energy neutron sources are known, and are used in measurement systems, such as in moisture detectors and interface level measurements. Fast neutrons may be converted to thermal neutrons when they travel in certain materials, especially hydrogen containing materials. Detection of either backscattered or transmitted thermal neutrons is the basis of such measurements. Thermal neutron detection systems are readily and inexpensively applied to hydrogenous process measurements. These detection systems are extremely limited in their detection of neutron energies above the thermal or epithermal range. The detection probability of these detection systems at above epithermal energies is only 1/100 of a percent of the capture probability at the thermal energies.
A system that uses direct measurement of the quantity of transmitted high energy or fast neutrons is the assignees Model 4360 Neutron Transmission System, the principles of which are disclosed in U.S. Pat. No. 4,794,256, titled FAST NEUTRON PROCESS MEASUREMENT SYSTEM.
The apparatus of the present invention detects backscattered neutrons having above epithermal energy, as opposed to measuring transmission of neutrons. A greatly reduced neutron source size can also be used in the present invention since the neutrons need not be transmitted through the entire vessel filled with the process material.
The present system measures density, level and interfaces of non-hydrogenous as well as hydrogenous materials based on neutron backscatter techniques. The invention expands the accurate measurement to materials that are either totally devoid of hydrogen or contain only trace amounts of hydrogen. Moderation of fast neutrons to thermal neutrons by hydrogen contained in the process material being monitored, is no longer a requirement and therefore not a limitation for the application of neutron backscatter to process measurements.
The invention allows cost effective application of neutron backscatter measurements to a broad new field of non-hydrogenous process materials. The ability to apply neutron backscatter instrumentation to process measurements that contain any element as opposed to only those that contain hydrogen substantially expands the use of the technology. Measurement error due to thermal neutron capture by high barns value elements is greatly reduced due to the energy detection spectrum of the invention.
Another benefit of the system in the present invention is that it is non-contacting and non-intrusive. Thus, it is not adversely affected by high temperatures or pressures, chemical corrosion, abrasion or other factors that adversely affect internal or contacting measurement systems.
When compared to thermal neutron backscatter systems, the present invention has significantly greater penetration through vessel walls. In thermal neutron systems interaction with the hydrogen contained in the process material readily reduces the neutrons to sub-epithermal ranges. After thermalization the neutron must traverse that portion of the process material that lies between the vessel and the point of thermalization, in addition to the vessel wall to reach the detectors. At thermal energies the maximum steel vessel wall thickness for process measuring is less than 3 inches. In the present invention, measurements can be made on vessels with a maximum thickness of 4 inches of steel or equivalent in refractory brick or other materials.
By way of comparison, thermal neutrons are considered to be neutrons having an energy level below 0.025 electron volts; epithermal neutrons have energy levels between 0.025 and 100 electron volts while the preferred fast neutrons emitted from the source with a mean energy level of over four million electron volts.
Another aspect of the system is that generally neutrons do not occur naturally as background radiation, thus it is practical to use small source sizes and detect the resulting low quantities of backscattered neutrons. Gamma radiation based systems must overcome a natural occurring gamma background radiation, which requires raising the total radiation level.